Dielectric heating system



1957 w. RUEGGEBERG DIELECTRIC HEATING SYSTEM Filed Sept. 29,

TO H. F. OSCILLATOR m E 8 F- G G E w ATTORNEY United States Patent Thisinvention relates to the art of dielectric heating. It is concernedparticularly with a system for simultaneously dielectrically heating aworkpiece or workpieces in a plurality of zones.

The invention will be described as applied to the heating pf a pluralityof spaced adhesive joints, for such a fabrication operation is typicalof the fields in which the invention will be useful.

In the manufacture of sealing gaskets of cork composition, for instance,dielectric heating may be utilized to 'activate an adhesive layerdisposed between the abutting edges of the end pieces and side pieces ofa generally rectangular frame-shaped valve cover gasket. In such anapparatus the electrodes will be positioned above and below theworkpieces over the joints, and the electric fields of force will bedirected generally along lines parallel to the adhesive line. Manualmanipulations involved in feeding the segments which form the gasket,etfecting the desired heating, and discharging the finished pieces areminimized if at least two and preferably all four of the joints are madesimultaneously. This poses the problem of obtaining uniform dielectricheating in all of the joints. There are many factors which affect suchheating. For instance, variations in the thickness of the adhesive line,variations in any air gaps which may exist between workpiece andelectrode, and variations in moisture content of the workpiece indifferent working areas adjacent to the adhesive line or therein, alldirectly affect the heating rate, resulting in nonuniform heating indifferent joints, and under aggravated conditions actually resulting inarcing between adjacent electrodes.

An object of the invention, therefore, is to provide for automatictemperature regulation in a multiple unit dielectric heating systemwhere the fields of force from a single high-frequency energy source areapplied to spaced areas of a workpiece or workpieces, avoidingobjectionable variations in temperature at the various heated areas andminimizing the problem of arcing.

There is a substantial problem of radiation of highfrequency waves fromcommercial installations where high-frequency electrical heating isemployed. Frequently, elaborate shielding is necessary to meet therequirements of the Federal Communcations Commission respecting theradiation of stray high-frequency fields, and this problem is aggravatedin systems where the electrodes are movable relative to the workpiece asin gasket assembly equipment.

Accordingly, another object of the invention is to provide a dielectricheating system in which high-frequency wave radiation will be minimizedinherently within the system.

An additional serious problem associated with dielectric bonding unitsis to minimize the hazards of accidental contact by operating personnelwith an electrically charged part of the system substantially aboveground potential. This frequently requires cumbersome safety guards andaccessories which present a substantial maintenance problem, anythingless than perfect operation being hazardous.

A further object of the invention is to provide a dielectric heatingsystem in which the exposed parts of the unit which might be engaged byoperating personnel are essentially at average ground potential.

In many dielectric heating systems it is necessary to provide forcircuit completion to ground through the electrodes. This reachesserious proportions in a multiple head dielectric bonding unit such asmight be used in gasket manufacture.

An additional object of the present invention is to provide for circuitcompletion in a dielectric heating system without a specificallyprovided ground return from the electrodes.

Other objects of the invention will become apparent from considerationof the following detailed description of an embodiment of the inventionwhich will be described in conjunction with the attached drawing, inwhich:

Figure l is a schematic diagram of a dielectric heating arrangementincluding a pair of bonding units;

Figure 2 is an isometric view showing a segmented, frame-shaped gasket;

Figure 3 is a diagrammatic sectional view showing the generalconstruction of a two-unit bonding arrangement with the electrodes inopen position; and

Figure 4 is a view similar to Figure 3 with the electrodes in closedposition.

In the embodiment illustrated, there are two pairs of electrodes 2 and 3and 4 and 5 between which workpiece 6 is disposed with itsadhesive-coated faces 7 and 8 disposed perpendicularly to the planes ofthe electrodes. In gasket assembly, the workpiece 6 might be made up ofan end segment and two side segments, and the coated faces 7 and 8 wouldbe disposed at the joints where the side segments abut the end segment.Preferred practice in gasket assembly is to coat with adhesive allabutting surfaces to be joined, and thus the coated faces 7 and 8 mayconsist of the two adhesive-coated surfaces of the end segment and theadhesive-coated surface of each of the side segments.

The electrodes 3 and 5 are coupled to a high-frequency alternatingcurrent source as noted in Figure 1. This source may be a ZOO-watt,65-megacycle high-frequency generator of conventional design andconstruction. An impedance matching network is provided within agrounded shielding case 9. It includes inductances 10 and 11 andvariable capacitances 12 and 13 connected to electrodes 3 and 5 and coil14 of transformer 15 fed by the high-frequency generator, with a groundconnection 16 between capacitances 12 and 13 at essentially theelectrical center of the circuit. Electrodes 2 and 4 are electricallyconnected by lead 17 and are capacitively coupled to electrodes 3 and 5,respectively, with the load 6 constituting a dielectric therebetween.While separate electrodes 2 and 4 have been illustrated, it will beclear that since they are electrically connected, a single electrodewhich is complementary to both electrodes 3 and 5 may be employed, ifstructurally more convenient. As will be noted from examination ofFigure 1, a bridge type circuit results and high-frequency current fromthe generator is forced around a loop starting at a, for instance,through the workpiece 6 and its adhesive line 7, through lead 17 andagain through workpiece 6 but now at its adhesive line 8, and thenending at b. The voltages at a and b are out of phase with respect topoint or connection 16 which is at ground potential. The electrodes 2and 4 are not grounded but are what may be termed floating secondaryelectrode means, with the electrodes 3 and 5 coupled to the source ofhigh-frequency alternating voltage constituting what may be termedprimary electrodes.

It will be clear from the foregoing that electrodes 2 and 4 and lead 17,which in actual commercial construction may constitute the movableelectrodes of the machine disposed above the assembly table, as will bedescribed more fully in conjunction with Figures 3 and 4, will have anaverage potential equal to potential at point 16 or ground. The voltagedrop within the load or workpiece 3 6 from electrode 3 to electrode 2,for example, may be from 2500 volts to essentially volt. The same wouldbe true of the voltage drop within the workpiece from electrode toelectrode 4, assuming the impedance of theworkpiece at adhesive lines '7and 8 and adjacent there-' to to be equal.

However, with the system of this invention, self-balancing oftemperatures at the two adhesive lines or joints is obtained because ofthe series configuration of the electrode circuit and the variableimpedance characteristics of each joint with respect to temperature. Amarked de crease in impedance results when the temperature of theadhesive and the cork composition increases. Thus any increase in thetemperature at one adhesive joint which is not equal to the temperatureincrease at the other joint results in a substantial difference inimpedance between the respective pairs of electrodes 2-3 and 45.Assuming, for instance, that the adhesive joint 7 between electrodes 2and 3 heats faster than the adhesive joint 8 between electrodes 4 and 5because of diflerences in adhesive line thickness, for example, itsheating rate automatically will be reduced through lower applied stress,since the majority of the voltage from point a to point b will beabsorbed at adhesive line 8 because of its higher impedance. Themajority of the applied voltage thus will be absorbed by the two jointsin such manner that the temperature of neither joint at any time willexceed greatly that which exists in the other.

In the diagram of Figure 1, the adhesive joints 7 and 8 have the samesurface area. In some segmented gaskets the adhesive joints to be formedmay be of nonuniform surface area; and, in such event, the division ofvoltage between the two may be adjusted by the variable capacitances 12and 13 to match the nonuniform impedances at the joints and, inoperation, the system will be self-compensating for any deviations inheating rates in the manner discussed above.

The apparatus shown in Figures 3 and 4 may be used in the assembly ofthe segmented frame-shaped gasket shown in Figure 2. The gasket is madeup of end pieces 18 and 19 and side pieces 20 and 21. All of theadhesivebonded joints 22, 23, 24, and may be activated simultaneouslywith a dielectric heating unit having two separate sources ofhigh-frequency power, or two of the joints may be activated in oneoperation and the other two in a subsequent operation. Figures 3 and 4show equipment for simultaneously forming two of the joints.

The unit comprises electrodes 26 and 27, each of which is receivedwithin an insulating holder 28 which may be made ofpolytetrafluoroethylene or other electrical insulating materialpossessing the requisite dielectric properties. The holder 28 isreceived within a metal frame 29 disposed within a work table 30 whichmay be made of reinforced phenolic resin or other insulating material.Screws 31 secure a metal impedance matching network case 32 to the table30, and a clamping arrangement 3334 with screws 35 which pass throughthe case 32 and into the electrode holder frame 29 holds each of theelectrode assemblies in fixed position. A lead-connecting screw 36extends from each electrode, and through these screws connection is madebetween the electrodes 26 and 27 and the impedance matching networksystem. A guiding frame 37 of polytetrafluoroethylene or other suitableinsulating material may be fastened to table 30 to aid in aligning thepieces to be joined.

The upper electrode system comprises a carrier 38 mounted on slides 39and 40 fixed to table 30. The carrier 38 is thus arranged for verticalreciprocation from the open or elevated position of Figure 3, in whichposition the. pieces to be joined may be inserted in their properrelationship for heat activation, to the closed position of Figure 4.There are shown two electrodes 41 and 42 which are complementary tolower electrodes 26 and 27 and which are adapted to overlie the joints22 and 23,

for instance, of gasket segments 18, 20, and 21. The two I electrodesare electrically connected by a conductor 43 which corresponds to theconnection 17 of Figure 1. Electrodes 41 and 42 are mounted inpolytetrafluoroethylene holders 44 or similar material, and the holdersare in turn secured to insulating frame members 45 attached to a plate46 secured to carrier 38. A carrier reciprocating member 47 is attachedto carrier 38 and may be mechanically, hydraulically, or otherwiseactuated to impart the desired vertical reciprocatory movements to thecarrier.

The floating secondary electrode and its associated reciprocatingmechanism are always at an average potential which is equal to groundpotential and thus these exposed parts of the machine, above theoperating table, are not hot and may be engaged by operating personnelwithout danger. This also minimizes the problem of radiation ofhigh-frequency fields. This radiation problem is also reduced by virtueof the phase shift between heating electrodes.

By having the electrode system in series configuration and in push-pullrelationship with the high-frequency source through the impedancematching network, the desired division of applied energy to the twojoints is obtained and self-balancing of temperature elevation in bothjoints consistently is assured. The system has a reduced a sensitivityto air gaps which may exist between workpiece and electrode, todifferences in adhesive thickness, and to the other variable factorswhich are frequently encountered in dielectric heating in multiple areaswith a single high-frequency source. Arcing tendencies through theworkpiece are also reduced to a point where they are of negligibleimportance. The complicated circuit completion arrangements to groundare not needed, for as a matter of fact no specific ground return isnecessary or provided. The exposed electrode or electrodes will be ataverage ground potential; and this, together with the 180 phase shiftbetween the heating electrodes, will minimize undesirable radiation.

I claim:

In a multiple-unit dielectric heating device, the combination of twoprimary electrodes, floating secondary electrode means capacitivelycoupled to said primary electrodes, said electrodes and said electrodemeans being arranged for the reception therebetween of material to bedielectrically heated in separate spaced areas, an impedance matchingbridge circuit comprising a variable reactance disposed in each of twoof the arms of said bridge, an inductance and the capacitance betweenone of said primary electrodes and said floating secondary electrodemeans disposed in each of the other two arms of said bridge, a groundconnection at essentially the electrical center between said reactances,and means inductively' coupling to said bridge across the arms thereof asource of alternating high-frequency electrical voltage to drive saidbridge, whereby the voltages applied to said primary electrodes aremaintained 180 out of phase with respect to ground as a result of thecircuitry of said bridge and said floating secondary electrode means ismaintained essentially at average ground potential.

References Cited in the, file of this patent UNITED STATES PATENTS2,109,323 Smith Feb. 22, 1938 2,298,038 Crandell Oct. 6, 1942 2,309,303Crandell Jan. 26, 1943 2,333,412. Crandell Nov. 2, 1943 2,474,420 HimmelJune 28, 1949 2,519,193 MacDermaid Aug. 15, 1950 2,542,702 Prow Feb. 20,1951 2,546,004 Kinn Mar. 20, 1951 2,551,757 Mittelmann' May 8, 19512,583,128 Stevenson et a1. Ian. 22, 1952 2,660,660 Von Hauteville Nov.24, 1953 2,662,162 Blok Dec. 8, 1953

